Acting Without
Thinking:
Separate Cortical Pathways for Visuomotor Control and Visual Perception
Melvyn A. Goodale
The University of Western Ontario
London Ontario N6A 5C2
mgoodale@uwo.ca
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Visual systems first evolved not to enable animals to see, but to
provide distal sensory control of their movements. Vision as 'sight' is
a relative newcomer on the evolutionary landscape, but its emergence has
enabled animals to carry out complex cognitive operations on
representations of the world. In the more ancient visuomotor systems,
the connections between the retina and motor pathways are relatively
direct. As a consequence, in these systems, there is a basic isomorphism
between visual input and motor output. In representational vision, there
are many cognitive �buffers� between input and output, and the
relationship between what is on the retina and the behaviour of the
organism cannot be understood without reference to other mental states,
including those typically described as "conscious". The duplex
nature of vision is reflected in the organization of the visual pathways
in the primate cerebral cortex. The dorsal 'action' stream projecting
from primary visual cortex to the posterior parietal cortex provides
flexible control of more ancient subcortical visuomotor modules for the
control of motor acts. The ventral 'perceptual' stream projecting from
the primary visual cortex to the temporal lobe provides the rich and
detailed representation of the world required for cognitive operations.
This might sound rather like Cartesian dualism�the existence of a conscious mind separate from a reflexive machine. But the division of labour between the two streams has nothing to do with the kind of dualism that Descartes proposed. Although the two kinds of visual processing are separate, both are embodied in the hardware of the brain. Moreover, there is a complex but seamless interaction between the ventral and the dorsal streams in the production of adaptive behavior. The selection of appropriate goal objects depends on the perceptual machinery of the ventral stream, while the execution of a goal-directed action is mediated by dedicated on-line control systems in the dorsal stream and associated motor areas. Moreover, as I will argue, the integration of processing in the two streams goes well beyond this. The dorsal stream may allow us to reach out and grasp objects with exquisite ease, but it is trapped in the present. Evidence from the behaviour of both neurological patients and normal observers shows that, by itself, the dorsal stream can deal only with objects that are visible when the action is being programmed. The ventral stream, however, allows us to escape the present and bring to bear information from the past � including information about the function of objects, their intrinsic properties, and their location with reference to other objects in the world. Ultimately then, both streams contribute to the production of goal-directed actions. Readings Milner, A. D., & Goodale, M.A. (2006). The Visual Brain in Action: 2nd Edition. Oxford: Oxford University Press, 297 pp. Goodale, M.A. & Milner, A.D. (2004). Sight Unseen: An Exploration of Conscious and Unconscious Vision. Oxford: Oxford University Press. 135pp.
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